The single zinc in primase from Escherichia Coli was studied using X-ray absorption spectroscopy (XAS). XAS provides information about the immediate sphere (radius about 5 ) of atoms surrounding the metal and has been widely used to characterize metalloproteins. The edge region of the spectrum provides information about the chemical identity of the ligating atom and their coordination geometry while the spectrum's fine structure provides information about the number of ligating atoms and their average distance from the metal. The structure of the zinc in several states of primase were determined. The zinc in free primase was found to be tetrahedrally ligated by three sulfurs and one histidine nitrogen located at a distance of 2.03 from the center of the metal. The zinc in high-magnesium conformation primase was ligated by two sulfurs, one histidine nitrogen and probably one oxyanion located at a distance of 2.03 . Finally, when ATP was added to primase, the zinc structure was latered to octahedral coordination by three sulfur, and three oxygen or nitrogens one of which could be a histidine nitrogen and another of which is probably an oxyanion. These results and others indicate that the primase zinc is very similar to the PMPS-removeable and nonessential B-site zinc from Escherichia Coli RNA polymerase which can also be coordinated by its initiating nucleotide ATP (Wu, F.Y.H., Huang, W.J., Sinclair, R.B., Powers, L. Journal of Biological Chemistry 267: 25560-25567, 1992. In light of this, the high-magnesium conformation result suggested that high magnesium, primase becomes inactive because the zinc is prevented from coordinating ATP.